Fuel cells are electrochemical energy conversion devices that generate electricity and heat by converting the chemical energy of fuels. A single fuel cell normally consists of an electrolyte sandwiched between two thin electrodes, a porous anode and a cathode. While a variety of differing fuel cells type have been developed, all operate on essentially the same principles. Hydrogen or a hydrogen-rich fuel is fed to the anode where a catalyst separates the hydrogen's negatively charged electrons from positively charged ions (protons). At the cathode, oxygen combines with protons and, in some cases with species such as water, resulting in water or hydroxide ions, respectively. For polymer exchanged membrane (PEM) and phosphoric acid fuel cells, protons move through the electrolyte to the cathode to combine with oxygen and electrons, producing water and heat. In other types of fuel cells such as solid oxide fuel cells (SOFCs), negative ions travel through the electrolyte to the anode where they combine with hydrogen to generate water and electrons. The electrons from the anode side of the cell cannot pass through the membrane to the positively charged cathode; they must travel around it via an electrical circuit to reach the other side of the cell. This movement of electrons is an electrical current which is advantageously used to drive a load, such as an electric motor or other electrical system.
In the case of hydrogen fuel cells, hydrogen fuel is fed to the anode in what is sometimes referred to as the anode loop. The quantity of hydrogen fed to the anode is a function of a variety of factors, including the relative purity of the hydrogen fuel, load demand, and other variable parameters that are unique to each fuel cell system application. Because of the varying parameters in the system, it is important to know the rate of mass flow of the hydrogen through the anode loop. Information concerning the mass flow rate of hydrogen may be used as a feedback signal to one or more controllers which operate and control various parts of the fuel cell system so as to satisfy the desired load demand with maximum efficiency. The need to measure the mass flow of hydrogen is made more difficult due to the presence of foreign substances such as water vapor and nitrogen mixed in the hydrogen which effectively have a dilutive effect.
Accordingly, there is a need in the art for a method and apparatus to accurately measure the mass flow of hydrogen in the anode loop of a fuel cell which is highly reliable and is capable of taking into consideration the dilutive effect of foreign substances in the fuel. The present invention is directed toward satisfying this need.